The dissertation is focused on the propagation of cosmic rays through the heliosphere. A short overview of topic history is included in the presentation. The description of selected methods to evaluate the distribution of cosmic rays in the heliosphere is presented. Special attention is pay to the estimation of statistical error of solution of Fokker–Planck equation by 1D forward stochastic differential equations method. The error dependence on simulation statistics and energy is presented for different combinations of input parameters. The 1% precision criterium in intensities and 1% criterium in standard deviation are defined as a function of solar wind velocity and diffusion coefficient value.